Nickel-aluminide based wear resistant material for piston rings

ABSTRACT

A piston ring comprises a coating of a wear-resistant composite composition, particularly intended for high temperature applications. The wear-resistant composite composition comprises a ceramic compound and at least 50 volume %, based on the total volume of the wear-resistant composition, of an intermetallic nickel-aluminide chosen from the group consisting of NiAl and Ni 3 Al. A piston ring comprises a wear-resistant composite composition, particularly intended for high temperature applications. The wear-resistant composite composition comprises a ceramic compound and at least an intermetallic nickel-aluminide chosen from the group consisting of NiAl and Ni 3 Al.

FIELD OF THE INVENTION

[0001] The invention relates to a piston ring comprising a coating of awear-resistant composite composition, particularly intended for hightemperature applications, the wear-resistant composite compositioncomprising a ceramic compound.

[0002] The invention further relates to a piston ring comprising awear-resistant composite composition, particularly intended for hightemperature applications, the wear-resistant composite compositioncomprising a ceramic compound.

TECHNICAL BACKGROUND

[0003] Very specific demands have to be met by high temperatureapplications e.g. piston rings that are intended for use in for instancemarine diesel engines, particularly as concerns strength, anti-corrosiveproperties, wear resistant, and material resilience. When used in adiesel engine the piston ring is arranged to abut on the one handagainst an associated piston groove, on the other against an enginecylinder-bore.

[0004] Consequently, the ring should be wear-resistant, particularly atthe interface towards the cylinder bore, where high friction isgenerated when the engine is in operation. The piston ring shouldtherefore also possess an inherent tension or resilience whereby thepiston ring will constantly be forced outwards, into abutment againstthe cylinder bore. In addition, upon each explosive stroke of theengine, the piston ring is urged with considerable force radiallyoutwards, into abutment against the cylinder bore, with consequentialincrease of stress. Due to a high working temperature in engines andespecially due to the impact of produced heat, from contact betweenpiston rings and cylinder liner during the process, many materials loosesome of their yield strength and show softening.

[0005] In operation, especially some contact areas between the pistonring and cylinder liner material are exposed to high temperatures, toconsiderable temperature differences, and to the effects of a highlycorrosive environment.

[0006] In order to withstand the effects of these stress-inducingcauses, the piston ring therefore also must exhibit considerable wearresistant, ductility, and thermal stability. By ductility is to beunderstood herein the maximum possible deformation of the materialbefore cracking begin.

[0007] Today, piston rings are generally manufactured from a cast-ironblank, which meets the requirements imposed on the material as regardsstrength and resilience but not on wear resistance on the surfacethereof that faces the cylinder bore. Cast iron does not possess therequired thermal stability at high temperature. A cast-iron piston ringblank therefore usually is provided with a wear-resistant wear layer onthe surfaces most exposed to wear.

[0008] The wear layer, which usually is formed by a chromium-compoundmaterial, is generally applied to the piston ring blank in anelectrolysis process as described e.g. in EP 0 668 375. In accordancewith the teachings of this specification the piston ring blank is givena hard chromium layer in an electrolysis process. However, difficultiesdo arise in achieving a sufficiently strong bond between the material ofthe blank and the material of the wear layer, which causes problems,because of the risk that the material of the wear layer be torn awayfrom the material of the blank. When this happens, the comparativelysoft material of the blank-material surface is exposed to wear in thearea of contact against the cylinder bore, with resulting considerableshortening of the life of the piston ring.

[0009] Another problem is that the coating gradually wears away, even ifthe bond between the surfaces is comparatively strong. The wear on thepiston ring progresses slowly as long as the wear layer is intact butvery rapidly, once that layer has disappeared. As a result, it may bedifficult to determine in time when a piston ring change should be made.

[0010] Another issue is to increase the oxidation resistance of thepiston ring at high temperatures. In WO9532314 there is provided anickel-aluminium intermetallic basis alloy considered suitable forpieces, such as gas turbine blades, exposed to a high and continuousthermal stress. This alloy is claimed to improve thermal, oxidation andthermo-chock resistance. However, the hardness properties andwear-resistance of this compound are considered insufficient for use byother pieces and elements exposed to wear and thermal stress.

SUMMARY OF THE INVENTION

[0011] The object of the present invention is to provide a material,particularly intended for medium to high temperature applications suchas piston rings, that meets the requirements necessary as regards wearresistance, resilience, anti-corrosiveness, hardness, thermal stabilityand ductility.

[0012] Another object is to provide a piston ring, which does not sufferfrom the above drawbacks found in the prior art. Other features andadvantages of the present invention will become apparent from thefollowing description of the invention.

[0013] The present invention provides a wear-resistant composition,intended for medium to high temperature applications, particularly forpiston rings, wherein the material comprises a composite ceramiccompound (hard phase). Said composite composition also comprises atleast an intermetallic nickel-aluminide chosen from the group consistingof NiAl and Ni3Al or a mixture thereof.

[0014] One of the advantages of the inventive wear-resistant compositionis the ability to withstand yield strength softening up to a temperatureof around 600° C. It is not unusual that a piston ring locally has towithstand temperatures in the range of 400 to 500° C. or more in workingconditions. If the metallic material (the matrix) in such a compositefor some reason, e.g. softening caused by high temperature, can notwithstand movement of the hard phase in the composite, the hard phase isnot likely to stay in place. The hard phase will leave their place andthen work like a polishing agent and cause higher wear (three bodywearing).

[0015] It is thus beneficial to use an intermetallic nickel-aluminidefor providing support to the ceramic compound in the composite material.An important object for the intermetallic nickel-aluminide is to keepthe hard phase in place during contact with liner material in order toavoid the earlier described polishing effect even in the above-mentionedtemperature range.

[0016] In a preferred embodiment of the present invention said compositecomposition comprises a matrix formed by the intermetallicnickel-aluminides from said group.

[0017] It is thus advantageous to use an intermetallic nickel-aluminidematrix for providing support to the ceramic compound in the compositematerial. An important object for the intermetallic nickel-aluminidematrix is to keep the hard phase in place.

[0018] The wear-resistant composition according to the invention alsocomprises a hard phase consisting preferably of at least a ceramiccompound chosen from a group consisting of aluminium oxide, chromiumcarbide, chromium oxide and silicon carbide e.g. Al₂O₃, Cr₃C₂, Cr₂O₃,and SiC.

[0019] The hard phase improves the wear resistance and the ability toresist thermal fatigue deformation of the composite. At the same timethe coefficient of thermal expansion of the inventive composition isreduced. The compatibility of hard phase and matrix is an importantfactor in this case. Al₂O₃ is preferred due to higher interface bondingwith the matrix material but the other hard phases are also working wellin this respect.

[0020] It is also an object of the present invention to provide a pistonring, particularly intended for diesel engines, consisting of theinventive wear-resistant composition.

[0021] It is also provided in accordance with the present invention apiston ring, particularly intended for diesel engines, wherein saidpiston ring comprises a coating of the inventive wear resistantcomposition. By using the wear-resistant composition especially ascoating on a substrate a more cost efficient product is obtained.

[0022] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent for oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof. Thus it isunderstood that various methods such as plasma-, HVOF spraying or otherrelated prior art methods can be used to apply the inventive compositionto a substrate such as e.g. a piston, a piston ring and a cylinder lineror at least parts of them.

BRIEF DESCRIPTION OF THE DRAWING

[0023] Currently preferred embodiments of the present invention will nowbe described in more detail, with reference to the accompanying drawing.

[0024]FIG. 1 is a schematic representation of superlattice dislocationsin a two-dimensional simple cubic lattice.

[0025]FIG. 2 is a table showing the rate of wear relative topressure-exposure in tests of a composition in accordance with oneembodiment of the invention vis-a-vis a well-known cast iron for pistonrings named Darcast.

[0026]FIG. 3 is a SEM-picture of a composition in accordance with theinvention.

[0027]FIG. 4 is one embodiment of a piston ring in accordance with thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] In accordance with a preferred embodiment of the invention it isprovided a wear-resistant composite composition, intended for medium tohigh temperature applications and particularly for piston rings, whereinthe wear-resistant composite composition comprises a ceramic mixture ofintermetallic composition and a ceramic compound. The wear-resistantcomposite composition specifically comprises at least one intermetallicnickel-aluminide chosen from the group consisting of NiAl and Ni₃Al. Theintermetallic nickel-aluminide or nickel-aluminides forms a matrix towhich a hard phase is added.

[0029] As used herein the term intermetallic compounds are a class ofmaterials, which can be viewed as occupying an intermediate positionregarding the properties of the material between metallic alloys andceramics. They are also considered as a unique class of metallicmaterials that form long-range ordered crystal structures below acritical temperature, generally referred to as the critical orderingtemperature (T_(c)).

[0030] The essential condition for a substitutional solid solution of asuitable composition to become ordered is that unlike atoms i.e.different elements must attract each other more than like atoms in orderto lower the free energy upon ordering. In other words, the degree oforder of intermetallic compounds is closely related to the nature of thebonding. These ordered intermetallics usually exist in relatively narrowcompositional ranges around simple stoichiometric ratio.

[0031] The ordered crystal structure of nickel aluminides makes itfeasible to achieve high tensile ductility. The ductility ofpolycrystalline Ni₃Al is further increased by micro alloying with boronaddition that segregates the grain boundaries and suppresses brittleintergranular fraction.

[0032] An above-mentioned effect depending on the crystal structure isrepresented schematically in FIG. 1 by a super lattice dislocation in atwo-dimensional simple cubic lattice. Intermetallic compounds exhibit avery high yield stress that is often maintained to elevatedtemperatures. Deformation in ordered alloys is controlled by the glideof super lattice or paired dislocations, as super dislocations, furtherillustrated in FIG. 1 for a two-dimensional ordered lattice having an ABcomposition.

[0033] The first, or leading, dislocation creates a layer of anti-phasedomain, which can be thought of simply as a layer of wrong bonding, andthe second, or following, dislocation restores the order. The relativelylow mobility of super dislocations gives higher yield strength; that is,yield strength increases rather than decreases with increasingtemperature.

[0034] The wear-resistant composition comprises a matrix formed by anintermetallic nickel-aluminide chosen from a group consisting of NiAland Ni3Al. The composition also comprises a ceramic compound chosen froma group consisting of chromium carbide, chromium oxide and aluminiumoxide e.g. Cr₂O₃, Cr₃C₂, SiC and Al₂O₃.

[0035] Examples of embodiments of the present invention are given below.

Example 1

[0036] A composition was prepared by forming a mixture of the followingingredients in the parts by weight quoted in Table 1: TABLE 1 No. NameElement Ni Al Cr Zr Nb 1 NiAl—Nb wt % 65-66 27-28 2-3 0.5-1 4-4.5

[0037] A powder was prepared by forming the mixture of components as setout in table 1 of example 1. Furthermore, 5-10% by volume of the initialmixture was replaced by 5-10% by volume of Al₂O₃. The resultingcomposition was heated and applied to a substrate in accordance withprior art techniques to form a wear-resistant composition on thesubstrate. The resulting composite had good wear resistance, ductility,and thermal stability properties.

[0038]FIG. 2 is a figure showing the rate of wear relative topressure-exposure in tests of a composition in accordance with oneembodiment of the invention vis-a-vis a well-known cast iron for pistonrings known as Darcast.

[0039] In order to further illustrate the properties of the invention,reference is made to FIG. 3, which is an SEM-picture of a preferredembodiment of a wear-resistant composition in accordance with theinvention. The picture discloses a matrix comprising darker “islands” ofa hardphase. In this case the hardphase is a chromium carbide. By meansof diffusion between matrix and hardphase a mixed zone, which is alsoclearly visible on the picture, is formed. The mixed zone provides forthat a hard bonding is formed between hardphase and matrix andpreferably the mixed zone is formed symmetrically around the hardphase.

[0040] A further embodiment of the wear-resistant composition accordingto the invention is outlined in the following example 2.

Example 2

[0041] TABLE 2 No. Name Element Ni Al Fe Mn Ti Zr B 2 Ni3Al— wt % 77-9-10 11- 0.5 0.5 0.01 O, 1 Fe 78 12

[0042] A powder was prepared by forming an initial mixture of componentsas set out in table 2 of example 2. Furthermore, 5-10% by volume of theinitial mixture was replaced by 5-10% by volume of Al₂O₃. The resultingcomposition was exposed to heat treatment and applied to a substrate inaccordance with e.g. prior art techniques to form a wear-resistantcomposition on a substrate. The resulting composite had good wearresistance, ductility, and thermal stability.

Example 3

[0043] A third example of a wear-resistant composition according to theinvention is given in the below table. TABLE 3 No. Name Element Ni Al CrC Mn Ti Zr B 3 Ni3Al—Cr wt % 81-83 8-9 7-8 0.1 0.4-0.5 1 0.6 0.1

[0044] A wear resistant composition was prepared by forming the mixtureof components as set out in Table 3 of example 3. In this examplechromium carbide is used as hard phase compared to the previous givenexamples. By, in accordance with the previous examples adding the hardphase Al₂O₃ to the mixture in table 3 instead, an increased thermalstability is obtained compared to the mixture according to table 3.Though Al₂O₃ is given as the preferred alternative for a hard phasecompound in the above stated examples, excellent results were achievedwith other hard phase compounds as well.

[0045] It is of course possible to use any hard phase from the groupconsisting of chromium carbide, chromium oxide, silicon carbide andaluminium oxide e.g. Cr₂O₃, Cr₃C₂, SiC and Al₂O₃ in combination with anycomposition given in the tables 1-3.

[0046]FIG. 4 is one embodiment of a piston ring in accordance with theinvention. The piston ring preferably comprises a coating of saidwear-resistant composition. In a second preferred embodiment of theinvention the piston ring consists of the inventive wear resistantcomposition.

[0047] As will be appreciated, the present invention is not limited tothe embodiments and examples described herein. For example, furthersubstances may be added to the wear-resistant composition in order tomodify its properties in some respect. Also other mixtures andcombinations than those listed in the tables 1-3 are possible within thescope of the invention. For special conditions up to 50%, preferablybetween 1-30%, and most preferably between 5-10% by volume of the totalcomposition can be hard phase such as, chromium oxide, silicon carbide,Cr₃C₂ and Al₂O₃.

1. A piston ring comprising a coating of a wear-resistant compositecomposition, particularly intended for high temperature applications,the wear-resistant composite composition comprising a ceramic compound,characterized in that said wear-resistant composition also comprises atleast 50 volume %, based on the total volume of the wear-resistantcomposition, of an intermetallic nickel-aluminide chosen from the groupconsisting of NiAl and Ni₃Al.
 2. A piston ring according to claim 1,wherein said composition comprises a matrix formed by an intermetallicnickel-aluminide chosen from said group consisting of NiAl and Ni₃Al. 3.A piston ring according to claim 1 or 2, wherein the intermetallicnickel-aluminide is a mixture of NiAl and Ni₃Al.
 4. A piston ringaccording any one of claims 1-3, wherein the ceramic compound is Cr₃C₂.5. A piston ring according to any one of claims 1-3, wherein the ceramiccompound is a chromium oxide, such as Cr₂O₃.
 6. A piston ring accordingto any one of claims 1-3, wherein the ceramic compound is SiC.
 7. Apiston ring according to any one of claims 1-3, wherein the ceramiccompound is aluminium oxide, such as Al₂O₃.
 8. A piston ring accordingto any one of claims 1-7, wherein the composition preferably comprisesbetween 70-99 volume % intermetallic nickel-aluminide based on the totalvolume of the wear-resistant composition.
 9. A piston ring according toany one of claims 1-7, wherein the composition preferably comprisesbetween 90-95 volume % intermetallic nickel-aluminide based on the totalvolume of the wear-resistant composition.
 10. A piston ring comprising awear-resistant composite composition, particularly intended for hightemperature applications, the wear-resistant composite compositioncomprising a ceramic compound, characterized in that said wear-resistantcomposition also comprises at least an intermetallic nickel-aluminidechosen from the group consisting of NiAl and Ni₃Al.
 11. A piston ringaccording to claim 10, wherein said composition comprises a matrixformed by an intermetallic nickel-aluminide chosen from said groupconsisting of NiAl and Ni₃Al.
 12. A piston ring according to any one ofclaims 10-11, wherein the intermetallic nickel-aluminide is a mixture ofNiAl and Ni₃Al.
 13. A piston ring according any one of claims 10-12,wherein the ceramic compound is Cr₃C₂.
 14. A piston ring according toany one of claims 10-12, wherein the ceramic compound is a chromiumoxide, such as Cr₂O₃.
 15. A piston ring according to any one of claims10-12, wherein the ceramic compound is SiC.
 16. A piston ring accordingto any one of claims 10-12, wherein the ceramic compound is aluminiumoxide, such as Al₂O₃.
 17. A piston ring according to any one of claims10-16, wherein the composition comprises at least 50 volume %intermetallic nickel-aluminide based on the total volume of thewear-resistant composition.
 18. A piston ring according to any one ofclaims 10-17, wherein the composition preferably comprises between 70-99volume % intermetallic nickel-aluminide based on the total volume of thewear-resistant composition.
 19. A piston ring according to any one ofclaims 10-17, wherein the composition preferably comprises between 90-95volume % intermetallic nickel-aluminide based on the total volume of thewear-resistant composition.